Effect of the end-groups upon delocalisation in polymethines: the first crystallographically characterised bond-alternated cyanine

The reorganisation energy associated with the end groups may be used to inf luence the localisation behaviour of cyanines; thus, the 1,3-bis(ruthenocen yl)allylium cation is markedly unsymmetrical in the crystal structure of it s hexafluorophosphate salt, with the positive charge localised on one ruthe nium, whereas the analogous iron species is delocalised and has approximate C-5 symmetry.